1. Field of the Invention
The invention relates to a control device for an internal combustion engine, the control device configured to control the controlled variables of an internal combustion engine by operating an ignition device including a spark plug provided in a combustion chamber of the internal combustion engine, and an ignition coil connected to the spark plug.
2. Description of Related Art
For example, Japanese Patent Application Publication No. 2013-24060) (JP 2013-24060 A) describes a device that increases a discharge current of a spark plug when the rate of airflow is adjusted to be equal to or higher than a prescribed value by an airflow control valve during execution of exhaust gas recirculation (EGR) control for introducing exhaust gas from an exhaust passage into an intake passage. Moreover, this device corrects the discharge current to a higher value as the ignition timing is advanced. Such a correction is made in view of the fact that, when the rate of airflow increases or the ignition timing is advanced, a discharge-current interruption, that is, a phenomenon in which a discharge current that flows between two electrodes (i.e., a pair of electrodes) of the spark plug is interrupted, is likely to occur due to an increase in the length of a path of the discharge current (refer to paragraph 0039 of JP 2013-24060 A).
When a discharge-current interruption occurs, the ignitability of an air-fuel mixture in a combustion chamber deteriorates. In view of this, if the occurrence of a discharge-current interruption can be reduced by the device, deterioration of the ignitability can be reduced. The present inventors have found that, in some operation states of an internal combustion engine, prolonging a current discharge duration is more effective than increasing the magnitude of a discharge current, in improving the ignitability of an air-fuel mixture in a combustion chamber. However, prolonging a current discharge duration in addition to increasing the magnitude of a discharge current causes an increase in the amount of energy consumed by an ignition device or creates the need for an increase in the thermal rating of the ignition device.